The present invention relates generally to modes of printing with swath-type printing systems. It relates more particularly to printmodes for improving the print quality of output produced by the printhead portion of an inkjet printer.
Inkjet printers, and thermal inkjet printers in particular, have come into widespread use in businesses and homes because of their low cost, high print quality, and color printing capability. The operation of such printers is relatively straightforward. In this regard, drops of a colored ink are emitted onto the print media such as paper or transparency film during a printing operation, in response to commands electronically transmitted to the printhead. These drops of ink combine on the print media to form the text and images perceived by the human eye. Inkjet printers may use a number of different ink colors. One or more printheads may be contained in a print cartridge, which may either contain the supply of ink for each printhead or be connected to an ink supply located off-cartridge. An inkjet printer frequently can accommodate two to four print cartridges. The cartridges typically are mounted side-by-side in a carriage which scans the cartridges back and forth within the printer in a forward and a rearward direction above the media during printing such that the cartridges move sequentially over given locations, called pixels, arranged in a row and column format on the media which is to be printed. Each print cartridge typically has an arrangement of printhead nozzles through which the ink is controllably ejected onto the print media, and thus a certain width of the media corresponding to the layout of the nozzles on the print cartridge, can be printed during each scan, forming a printed swath. The printer also has a print medium advance mechanism which moves the media relative to the printheads in a direction generally perpendicular to the movement of the carriage so that, by combining scans of the print cartridges back and forth across the media with the advance of the media relative to the printheads, ink can be deposited on the entire printable area of the media.
The quality of the printed output is a very important feature to purchasers of inkjet printers, and therefore manufacturers of inkjet printers pay a great deal of attention to providing a high level of print quality in their printers. Aberrations in the printhead nozzles can undesirably reduce print quality; such aberrations include, for example, not ejecting ink at all, ejecting an incorrect volume of ink in a drop, producing irregularly shaped drops with artifacts such as tails, or producing a spray of extraneous droplets in addition to the desired drop. Another common type of nozzle aberration is directionality error, also known as dot placement error, in which the drops of ink are not precisely printed in the intended locations on the print media. Different types of printheads can exhibit different types of dot placement errors; these errors are typically due to the design of the printhead and can be characterized for printheads of that particular type. In some types of printheads, it is common for the nozzles located at the top and bottom ends of the printhead to exhibit significant dot placement errors in the direction along the media advance axis, resulting in errors in the swath height, while the nozzles located in the middle of the printhead exhibit less dot placement error. Because the error-prone nozzles print the top and bottom edges of the printed swath in the wrong place, a visually significant print quality defect known as banding results. Banding results in strip-shaped nonuniformities that are visible throughout the printed image.
Banding is more objectionable in areas of the image that contain midtones, rather than highlights (light) or saturated (dark) areas. Dot placement errors are difficult to see in an area of highlights because there is typically so much white space (unprinted areas of the print medium) between the drops of ink that the placement errors are not readily perceived by the human eye. Saturated areas do not exhibit much banding because they contain very little white space, and the large volumes of ink placed in these areas hide most placement errors. But in midtone areas, which have moderate amounts of both white space and ink, small errors in dot placement can have a large effect on how much white space a person perceives.
To minimize banding due to dot placement errors (and coincidentally to also reduce the effect of printing defects resulting from having too much ink on the print medium at one time, such as bleeding of one color area into another and warping or wrinkling of the print media), most printers do not print all the required drops of all ink colors in all pixel locations in the swath in one single scan, or xe2x80x9cpassxe2x80x9d, of the printheads across the media. Rather, multiple scans are used to deposit the full amount of ink on the media, with the media being advanced after each pass by only a portion of the height of the printed swath. In this way, areas of the media can be printed in on more than one pass. In a printer which uses such a xe2x80x9cmultipassxe2x80x9d printing mode, only a fraction of the total drops of ink needed to completely print each section of the image is laid down in each row of the printed medium by any single pass; areas left unprinted are filled in by one or more later passes. When printing of a page is complete, every area of the print medium has typically been printed on by the same multiple number of passes. Because each pass uses a different nozzle to print a particular row of the image, multipass printing can compensate for nozzle defects. To illustrate how this compensation works, consider the defect where one particular nozzle in a single-pass printmode does not work at all, causing an unprinted row (or band) of unprinted pixel locations to appear in the printed image. However, if a four-pass printmode is used instead of a single pass, the defective nozzle will only print one out of every four drops in that row, making the impact of the defective nozzle less objectionable. While the above example, for illustration, used a broken nozzle, the same principle applies to nozzles with directionality errors which print ink at incorrect locations.
However, a multipass printmode where all nozzles can deposit the same number of drops of ink is often insufficient to improve print quality to an acceptable level, particularly when specific groups of nozzles have worse errors than other groups, as in the case of swath height error as described above. Therefore, some other approaches to improving print quality have modified the printmode such that all nozzles no longer print the same number of ink drops. For instance, a printmode which prints with only the middle nozzles of a printhead which exhibits swath height error results in improved print quality. However, such an approach has the drawback of significantly increasing the amount of time it takes to print a page, because a smaller swath is printed on each pass.
Other printmodes improve print quality by printing using all nozzles, but depositing fewer drops from the end nozzles than the middle nozzles. Examples of such printmodes are described in the co-pending and commonly-assigned European patent application Ser. No. 99301151.9, by Vinals, filed Feb. 17, 1999, titled xe2x80x9cPrinting Apparatus and Methodxe2x80x9d, which is hereby incorporated by reference in its entirety. Other such examples are described in the above-referenced copending and commonly-owned U.S. patent application Ser. No. 09/399,473 by Askeland titled xe2x80x9cBanding Reduction in Multipass Printingxe2x80x9d. Because some nozzles print less than other nozzles in these printmodes, all nozzles do not have the same xe2x80x9cprint loadxe2x80x9d. As a result, in many cases the useful life of the printhead will be reduced, because kogation (a buildup of residue on the surface of a nozzle""s firing resistor), cavitation damage (physical damage to the surface of the nozzle""s firing resistor), and other detrimental effects related to the number of times a nozzle deposits ink drops will adversely affect the more frequently used nozzles sooner than would occur if all nozzles had an equal print load.
Just as important to an inkjet printer purchaser as print quality is the useful life of the printhead. In order to maintain print quality, printheads are often replaced by users once any nozzle or group of nozzles degrades to a point where print quality is adversely affected. Accordingly, there is still a need for an inkjet printer that minimizes print quality defects due to nozzle aberrations but without significantly reducing the useful life of the printhead.
In a preferred embodiment, the present invention provides a multipass swath printing system that improves the quality of the printed output by enabling lower-quality nozzles with a greater tendency toward dot placement errors to deposit into a specific pixel location a small number of drops in each of several printing passes, and enabling higher-quality nozzles with a lesser tendency toward dot placement errors to deposit into a specific pixel location many drops rapidly during at least one of the printing passes. Because the number of drops enabled for deposition from all nozzles is substantially identical, such a printing system provides improved print quality without a significant reduction in useful life of the printhead.
An embodiment of the printing system according to the present invention includes a printhead mounted in a carriage which is attached to a frame for relative motion with respect to a print medium. The printhead has an arrangement of nozzles through which ink is ejected onto a pixel grid of multiple rows on the print medium when the carriage makes a printing pass. The system also contains a print controller which activates the nozzles to deposit the ink onto the medium, as governed by a printmask. The printmask defines predetermined groups of nozzles to be activated to deposit ink at specific locations in the pixel grid during multiple printing passes of the printhead over the print medium, and the amount of ink to be deposited into the specific locations, based on statistical or theoretical determination of expected dot placement error from the nozzles. The printmask has a xe2x80x9chi-fipexe2x80x9d mask subpattern associated with a first group of the nozzles enabling each nozzle to deposit a small number of drops (typically one) into specific locations during each of several of the passes, and a xe2x80x9cmultidropxe2x80x9d mask subpattern associated with a second group of the nozzles enabling each nozzle to many drops (typically two to four) rapidly into specific locations during at least one of the passes. In printheads which are linearly extended along the media advance axis, the first group of nozzles is substantially near ends of the printhead, and the second group of nozzles is substantially away from the ends of the printhead. In a preferred embodiment, the printmask also has a composite mask subpattern associated with a third group of the nozzles enabling each nozzle to both deposit a small number of drops into specific locations during each of several of the passes, and many drops rapidly into specific locations during at least one of the passes. The third group of nozzles is disposed between the first and second groups, and the mask subpattern is preferentially graduated such that the small number of drops predominate for nozzles adjacent the first group, and the many drops predominate for nozzles adjacent the second group.
More than one printhead may be included in the swath printing system. In some embodiments, all the printheads are aligned relative to each other such that each printhead deposits the ink in substantially an identical swath in a single pass. In an alternate embodiment, the printheads are offset relative to each other such that each printhead deposits the inks in substantially a different swath in a single pass. In still another embodiment, the printheads are partially aligned relative to each other such that two adjacent printheads deposit the inks in an overlapping swath in a single printing pass. Each printhead may be mounted in an individual cartridge, or multiple printheads may be mounted in a same cartridge, such as a tri-color cartridge containing three printheads. The three printheads may be aligned relative to each other such that the printheads deposit the inks in substantially an identical swath in a single printing pass, or may be offset relative to each other such that the printheads deposit the inks in different swaths in a single printing pass.
The present invention may also be implemented as a method for incremental printing with an inkjet swath printer. The method preferably includes establishing a pixel grid on a print medium by depositing drops of ink in multiple printing passes of a printhead over the print medium in a scan direction and advancing the medium in a media advance direction between at least some of the multiple printing passes, and printing into a specific pixel of the grid by depositing both a small number of drops in each of several of the printing passes and many drops rapidly during at least one of the printing passes. Regardless of the printing mode used for different nozzles, all nozzles are enabled to print substantially the same amount of ink into the pixel grid. In printheads which are linearly extended along the media advance axis, the first group of nozzles is substantially near ends of the printhead, and the second group of nozzles is substantially away from the ends of the printhead.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.